A circuit with adjustable admittance is known from U.S. Pat. No. 4,109,214. This publication describes the parallel circuit as a differential pair, with a first and second transistor whose emitters are connected to each other and to a DC current source. The admittance element, for example a capacitor, is connected between the collector and the emitter of the first transistor. The first terminal is connected to the collector of the first transistor. The collector of the second transistor is coupled to the second terminal, which for AC purposes may be regarded as circuit ground. In operation the emitter connection will have a low effective impedance to ground; this means that for the purpose of determining the current through the admittance element this element may be regarded as connected between the first and second terminal.
When an alternating voltage is supplied across the terminals a current will develop through the admittance element. A part of this current will be forcibly supplied back to the first terminal via the collector of the first transistor. The remainder of the current flows via the second transistor to the second terminal. This remainder is equal to the net current that is drawn from the first terminal and is smaller than the current through the admittance element.
As a consequence, the effective admittance has an admittance value which can be adjusted by controlling the conduction through the first and second transistor. The effective admittance value can be adjusted between zero admittance (in case the first transistor is made completely conductive and the second transistor is made non-conductive) and the admittance of the admittance element (in case the first transistor is made non conductive and the second transistor is made completely conductive).
The conduction through the first and second transistor (and with it the effective admittance) is controlled by the difference between the voltages on the base connections of the first and second transistor. The effective admittance value depends exponentially upon this difference. The prior art teaches a control circuit which is coupled between the input for the control signal M and the base connection, and which realizes a linear relation between the effective admittance value and the control signal M.
However, the adjustment of the effective admittance is often used in order to adjust an electronic parameter of the circuit other than the effective admittance itself. In such a case the linear relation between the effective admittance value and the control signal M does not guarantee a linear relation between the control signal M and the electronic parameter.